Generally, exhaust gas of vehicles contains a large amount of harmful substances such as carbon monoxides, nitrogen oxides, and hydrocarbon Particularly, the production rate of harmful substances such as nitrogen oxides increase as the temperature of an engine is increased.
Nowadays, exhaust gas regulations in each country are being reinforced. To meet such reinforced exhaust gas regulations of each country, an exhaust gas recirculation (EGR) apparatus is provided in a vehicle as a means for reducing harmful substances such as nitrogen oxides contained in exhaust gas.
The EGR apparatus supplies some exhaust gas of the vehicle along with mixing air into a combustion chamber of the engine, thus reducing the temperature of the combustion chamber, thereby reducing a discharge rate of harmful substances such as nitrogen oxides or sulfur oxides.
To achieve the above-mentioned purpose, the EGR apparatus includes an exhaust gas cooler (EGR cooler) which reduces the temperature of exhaust gas to be drawn into the combustion chamber so that the temperature of exhaust gas discharged from the combustion chamber can be reduced to a predetermined temperature before the exhaust gas is drawn into the combustion chamber.
Examples of a conventional exhaust gas cooler were proposed in Korean Patent Unexamined Publication No. 10-2012-0121224 and US Patent No. 2013-0213368.
Referring to Korean Patent Unexamined Publication No. 10-2012-0121224, an exhaust gas cooler in accordance with a first conventional art includes a heat exchange pipe which cools exhaust gas using cooling water of an engine. The heat exchange pipe is configured such that exhaust gas passes through the heat exchange pipe in one direction. A heat dissipation fin is provided in the heat exchange pipe so that a heat exchange area of exhaust gas in the heat exchange pipe can be increased.
Referring to US Patent No. 2013-0213368, an exhaust cooler in accordance with a second conventional art includes a heat exchange pipe which cools exhaust gas using cooling water of an engine. The heat exchange pipe is configured such that, to increase the length of an exhaust gas flow passage, the flow direction of exhaust gas drawn into the heat exchange pipe in one direction can be changed to the opposite direction before the exhaust gas is discharged out of the heat exchange pipe.
However, the conventional exhaust gas coolers are problematic in that heat exchange performance (cooling performance for cooling exhaust gas) is reduced in a confined space. In detail, the exhaust gas cooler according to the first conventional art includes the heat dissipation fin for enhancing the heat exchange performance, but because the heat dissipation fin cannot have a bent structure, the heat exchange pipe must be formed to extend in one direction. That is, an inlet and an outlet of the heat exchange pipe are open in opposite directions on the same axis, and a flow passage communicating the inlet and the outlet of the heat exchange pipe with each other is formed in a linear direction. Therefore, the length of the exhaust gas flow passage in the heat exchange pipe is comparatively short, and the heat exchange performance is reduced. On the other hand, in the exhaust gas cooler according to the second conventional art, to increase the length of the exhaust gas flow passage in the heat exchange pipe and enhance the heat exchange performance, the heat exchange pipe is configured such that the flow direction of exhaust gas drawn into the heat exchange pipe in one direction can be changed to the opposite direction before the exhaust gas is discharged out of the heat exchange pipe. In other words, the inlet and the outlet of the heat exchange pipe are open in the same direction. A flow passage communicating the inlet and the outlet of the heat exchange pipe with each other is formed to extend from the inlet of the heat exchange pipe in one linear direction, bend along a semicircular line, extend from the bent portion in one direction, and communicate with the outlet of the heat exchange pipe. However, since the flow passage is rapidly changed in direction, pressure drop of exhaust gas is increased (a difference between a pressure of exhaust gas in the inlet of the heat exchange pipe and a pressure of exhaust gas in the outlet of the heat exchange pipe is increased), whereby the heat exchange efficiency is reduced. Furthermore, because the heat exchange pipe is bent, a separate heat dissipation fin cannot be provided in the heat exchange pipe. As a result, the improvement in the heat exchange performance is limited.